This invention relates to a control apparatus for an elevator which is driven by an induction motor.
There has been an elevator system wherein an induction motor is employed as an electric motor for driving a cage, and it is subjected to a slip frequency control, thereby to operate the cage with the torque of the motor controlled. FIG. 2 is a block diagram of an example of a prior-art control apparatus disclosed in the official gazette of Japanese Patent Application Laid-open No. 59-17879.
Referring to the figure, numeral 1 designates a three-phase A.C. power source terminal, and numeral 2 an electromagnetic contractor contact which is connected to the three-phase A.C. power source terminal and which closes at the start of a cage 9 to be described below and opens at the stop thereof. Numeral 3 indicates a converter by which a three-phase alternating current obtained through the electromagnetic contactor contact 2 is changed into direct current, numeral 4 a smoothing capacitor which is connected across the output terminals of the converter 3, and numeral 5 an inverter of the pulse width modulation system which is connected to both the terminals of the smoothing capacitor 4 and which changes a fixed D.C. voltage into alternating current of variable voltage and variable frequency. A three-phase induction motor (hereinbelow, termed `induction motor`) 6 is driven by the inverter 5, the drive sheave 7 of a hoist is driven by the induction motor 6, a main rope 8 is wound round the drive sheave 7, the cage 9 and a counterweight 10 are respectively coupled to both the ends of the main rope 8, and a tachometer generator 11 detects the revolution speed of the induction motor 6 to generate a speed signal 11a. An adder 13 subtracts the speed signal 11a from a speed command signal 12, and delivers a deviation signal. Connected to the adder 13 is a compensating element 14 which serves to improve the response of a speed control loop, which has a transfer function G(s) and which provides a slip frequency command signal 14a as its output. An adder 15 adds the slip frequency command signal 14a and the speed signal 11a. A voltage command generator 16 generates a voltage command signal 16a by receiving the output 15a of the adder 15, while a frequency command generator 17 generates a frequency command signal 17a similarly. A switching device 18 brings contact pieces 18a and 18b into touch with the sides of contacts a respectively when the slip frequency command signal 14a is plus or zero, and it brings them into touch with the sides of contacts b respectively when the slip frequency command signal 14a is minus. A gain changer 19 is connected to the contact b with which the contact piece 18a is switchedly brought into touch, and upon receiving the output 15a of the adder 15, it produces an output of a set value in accordance with the received value. A power control device 20 is connected to the contact b with which the contact piece 18b is switchedly brought into touch, and upon receiving the speed signal 11a, it produces a frequency command signal (fixed value) with which regenerative power to be regenerated from the induction motor 6 to the D.C. side becomes null. An inverter control device 21 controls the output voltage and output frequency of the inverter 5 on the basis of the voltage command signal 16a, the frequency command signal 17a, the output of the gain changer 19, and the frequency command signal of the power control device 20.
The prior-art control apparatus for the elevator is constructed as described above. Thus, during the power operation of the induction motor 6, the slip frequency command signal 14a which is evaluated with the deviation signal between the speed command signal 12 and the speed signal 11a is plus, so that the contact pieces 18a and 18b are respectively held in touch with the sides of the contacts a as shown in the figure. Accordingly, the slip frequency command signal 14a and the speed signal 11a are added by the adder 15, and the resulting output signal 15a is input to the voltage command generator 16 and the frequency command generator 17. In the voltage command generator 16 and the frequency command generator 17, the voltage command signal 16a and the frequency command signal 17are respectively generated according to which the output voltage/output frequency satisfy a substantially fixed relationship. On the basis of these command signals, the inverter control device 21 controls switching elements constituting the inverter 5 and causes the induction motor 6 to generate a torque corresponding to the slip frequency command signal 14a.
Meanwhile, in the elevator, when the cage 9 is decelerated and stopped, mechanical energy is converted into electrical energy through the induction motor 6, and regenerative power is fed back to the D.C. side through the inverter 5. On this occasion, when the aforementioned control making the output voltage/output frequency substantially constant is performed, the regenerative energy is stored in the smoothing capacitor 4 to raise the terminal voltage thereof, and this capacitor 4 itself and the inverter 5 might be destroyed.
Therefore, in the mode of the regenerative operation of the induction motor 6, the switching device 18 switches the contact pieces 18a and 18b to the sides of the contacts b upon detecting that the slip frequency command signal 14a has become minus. Thus, the slip frequency command signal 14a is directly input to the gain changer 19, the output of which is applied to the inverter control device 21 as a voltage command signal. Besides, the power control device 20 receives the speed signal 11a as its input, and it creates a frequency command signal making the regenerative power null and applies this signal to the inverter control device 21. The expression `control making the regenerative power null` signifies none other than consuming all the mechanical energy within the motor. The principle of the power control device 20 which generates the frequency command signal for this purpose will be described by referring also to the equivalent circuit of the induction motor shown in FIG. 3.
Referring to FIG. 3, electric power P.sub.1 which is consumed within the induction motor 6 is given by: ##EQU1## Here, V: A.C. input voltage,
Z: overall impedance of the induction motor 6, PA1 g.sub.0 : excitation conductance, PA1 r.sub.1, r.sub.2 : primary resistance and secondary resistance (calculated into a primary value) of the induction motor 6, PA1 x.sub.1, x.sub.2 : primary leakage reactance and secondary leakage reactance (calculated into a primary value) of the induction motor 6, PA1 s: slip of the induction motor 6.
On the other hand, electric power P.sub.2 which is generated as regenerative power is given by: ##EQU2## Here, when the slip s is controlled so as to hold: EQU P.sub.1 +P.sub.2 =0 (4)
all the mechanical energy is consumed within the induction motor 6.
Substituting Eqs. (1) and (3) into Eq. (4), ##EQU3## where Z=Z(s) is put. By evaluating s which satisfies Eq. (5) without regard to the input voltage V, the slip s which gives rise to a braking force without the exchange of electric power is obtained. Further, when the speed signal 11a is given, the frequency command signal for the inverter 5 is determined. It is accordingly understood that only the speed signal 11a may be given to the power control device 20. Symbol b.sub.0 in FIG. 3 denotes an excitation susceptance.
As thus far described, in the power operation mode of the induction motor, the prior-art control apparatus for the elevator as shown in FIG. 2 controls the torque with the slip frequency command signal corresponding to the deviation between the speed command signal and the speed signal, while in the regenerative braking mode, it controls the frequency command signal to be applied to the inverter, with the speed signal and makes the regenerative power of the induction motor null, thereby to protect the smoothing capacitor 4 and the inverter 5.
In the prior-art control apparatus for the elevator, however, the power control device 20 sets the slip s conforming to the aforementioned equation (5) on the basis of the predictive values of the primary resistance r.sub.1 and secondary resistance r.sub.2 of the induction motor. This had led to the problem that the values of these resistances fail to agree with the initial predictive values on account of the temperature rise of the induction motor, so the regenerative power cannot be held null at all times.